In Integrated Circuit (IC) manufacturing individual devices, such as the transistors, are fabricated in the silicon substrate and then they are connected together to perform the desired circuit functions. This connection process is generally called “metallization”, and is performed using a number of photolithographic patterning, etching, and deposition steps.
The deposition of tungsten (W) films using chemical vapor deposition (CVD) techniques is an integral part of many semiconductor fabrication processes since it can produce low resistivity electrical connection between i) adjacent metal layers (vias) and ii) first metal layer and the devices on the silicon substrate (contact). Typically the W film is deposited through the reduction of tungsten hexafluoride (WF6) by hydrogen (H2) or silane (SiH4). In a typical tungsten process, the wafer is heated to the process temperature in a vacuum chamber, and them soaked in SiH4 gas to protect the already deposited titanium liner thin film on the substrate from possible reaction with WF6. A thin layer of tungsten film, known as the seed or nucleation layer, is deposited by the reaction of WF6 and SiH4. Finally, the via or contact is filled with tungsten by the reaction of WF6 and H2 (“plugfill”).
Conventionally, the WF6 and reducing gas (SiH4 or H2) are simultaneously introduced into the reaction chamber. It is expected that in a tungsten process all vias and contacts are completely filled with tungsten material, i.e., a 100% plugfill is achieved. The tungsten plugfill process is very sensitive to the conformality of the tungsten seed or nucleation layer in the vias and contacts.
The common problems associated with many seed layer deposition techniques are poor sidewall step coverage and conformality. This means that the seed layer is much thicker in wide-open areas, such as on top of the contacts and vias as compared to the bottom and lower portion of the sidewalls of the contacts and vias. With the decrease of the design rule of semiconductor devices, the diameter of the contacts and vias get smaller while their heights do not decrease. Thus, the aspect ratio (height divided by diameter) of contacts and vias keep increasing. The increased aspect ratio exacerbates the problem with the step coverage and conformality, thus degrading the quality of the plugfill process.
In a conventional CVD process, reactive gases arrive at the substrate simultaneously with film growth resulting from continuous chemical reaction of the precursor and reactant gases on the substrate surface. Uniform and reproducible growth of the film is dependent on maintenance of the correct precursor and reactant flux at the substrate. The growth rate is proportional to the precursor flux at the substrate and to the substrate temperature.
Atomic layer deposition (ALD) is a method of sequentially depositing a plurality of atomic layers on a semiconductor substrate by sequentially injecting and removing reactants into and from a chamber. ALD is a surface controlled process and uses two-dimensional layer by layer deposition. ALD uses a chemical reaction like CVD but it is different from CVD in that reactant gases are individually injected in the form of a pulse instead of simultaneously injecting reactant gases, so they are not mixed in the chamber. For example, in a case of using gases A and B, gas A is first injected into a chamber and the molecules of gas A are chemically or physically adsorbed to the surface of a substrate, thereby forming an atomic layer of A. The gas A remaining in the chamber is purged using an inert gas such as argon gas or nitrogen gas. Thereafter, the gas B is injected and chemically or physically adsorbed, thereby forming an atomic layer of B on the atomic layer of A. Reaction between the atomic layer of A and the atomic layer of B occurs on the surface of the atomic layer of A only. For this reason, a superior step coverage can be obtained regardless of the morphology of the substrate surface. After the reaction between A and B is completed, residuals of gas B and by products of the reaction are purged from the chamber. The process is repeated for multiple layers of material to be deposited.
Thus, in contrast to the CVD process, ALD is performed in a cyclic fashion with sequential alternating pulses of the precursor, reactant and purge gases. The ALD precursor must have a self-limiting effect such that the precursor is adsorbed on the substrate in a monolayer atomic thickness. Because of the self-limiting effect, only one monolayer or a sub-monolayer is deposited per operation cycle. ALD is conventionally conducted at pressures less than 1 Torr.
Methods, which would form uniform seed layers in channel or vias and result in an improvement in the subsequent filling of the channel or vias by conductive materials, has long been sought, but has eluded those skilled in the art.